There were 15 people riding on a train before it stopped and some number of people got off. Now there are 6 people riding on the train. How many people got off the train at the stop?
Answer: The number of people who got off is the difference between the number who were riding before and the number who are riding now. The difference is $15 - 6$ people. $15 - 6 = 9$ people got off.